Conventional MEMS elements are disclosed in e.g. Patent Documents 1 and 2 listed below. The MEMS element disclosed in Patent Document 1 will be described below with reference to FIGS. 18 and 19 of the present application, and the MEMS element disclosed in Patent Document 2 will be described below with reference to FIGS. 20 and 21.
Patent Document 1: JP-A-2004-12327
Patent Document 2: JP-A-2005-40885
The conventional MEMS device X shown in FIG. 18, structured as a capacitance type acceleration sensor, includes a substrate 91 on which an MEMS element Y and controllers 95a and 95b are mounted. The substrate 91, the MEMS element Y and the controllers 95a and 95b are covered by a resin package 96. A plurality of external terminals 97 for mounting project from the resin package 96.
FIG. 19 is an exploded perspective view of the MEMS element Y. As shown in the figure, the MEMS element Y includes a pair of glass substrates 92a and 92c, and an Si substrate 92b which are laminated. A movable electrode 94 comprising a movable portion 94a and a support portion 94b is incorporated in the Si substrate 92b. The MEMS element Y including such an Si substrate is generally called an MEMS element by bulk micromachining.
The paired glass plates 92a and 92c are formed with fixed electrodes 93a and 93b, respectively. The movable portion 94a and the fixed electrode 93a face each other, thereby forming a variable capacitor. Similarly, another capacitor is formed between the movable portion 94a and the fixed electrode 93b. When acceleration occurs in the vertical direction in FIG. 19, the movable electrode 94 is relatively moved up or down, whereby the capacitances of the two capacitors will vary. Based on the variation of the capacitances, the MEMS device X determines the magnitude of the acceleration.
The MEMS device X may be mounted to a cellular phone provided with a HDD as the recording medium. With such an arrangement, when the cellular phone is overaccelerated, the MEMS device X detects the acceleration. For instance, when the user drops the cellular phone onto the floor, the MEMS device X can detect the fall. Thus, it is possible to arrange that, based on such detection, the rotation of the HDD may be stopped, or the data read/write head may be moved away from the HDD.
To utilize the MEMS device X in the above-described manner, the MEMS device X needs to have a small thickness to be able to be mounted to the cellular phone. However, the MEMS device X is relatively thick because of the employment of the MEMS element Y processed by the bulk micromachining. Specifically, the Si substrate 92b of the MEMS element Y incorporates the movable electrode 94 by chipping a relatively thick Si material. Thus, the MEMS element Y comprising the lamination of the Si substrate 92b and the glass substrates 92a, 92c has a relatively large thickness. Since the MEMS device X further includes a resin package 96 for sealing the MEMS element, the thickness of the MEMS device X further increases. Thus, the MEMS device X cannot meet the demand for thickness reduction.
As shown in FIG. 20, the MEMS element disclosed in Patent Document 2 includes a substrate 91′, and a lower electrode 92′ and a wiring 95′ formed on the substrate. An upper electrode 93′ is connected to the wiring 95′. The upper electrode 93′ includes a movable portion 93a′ spaced from the lower electrode 92′ in the vertical direction in the figure and facing the lower electrode.
When the MEMS element is used for an acceleration sensor, the magnitude of the acceleration in the vertical direction in FIG. 20 is measured based on the variation of the capacitance between the lower electrode 92′ and the upper electrode 93′. Specifically, the capacitance between the lower electrode 92′ and the upper electrode 93′ depends on the size of the gap between the lower electrode 92′ and the movable portion 93a′ of the upper electrode 93′. When acceleration in the vertical direction is produced, the movable portion 93a′ is relatively moved in accordance with the acceleration, causing the size of the gap to vary. As a result, the capacitance varies. By electrically measuring the variation of the capacitance, the magnitude of the acceleration can be determined.
FIG. 21 shows a step in the process of manufacturing the MEMS element of FIG. 20. In this manufacturing process, a sacrifice layer 94′ is formed to cover the lower electrode 92′, and a movable portion 93a′ is formed on the sacrifice layer 94′. Then, by removing the sacrifice layer 94′, a gap of a predetermined dimension is defined between the lower electrode 92′ and the movable portion 93a′. The removal of the sacrifice layer 94′ may be performed by wet etching using an etchant which dissolves only the sacrifice layer 94′.
To enhance the measurement accuracy of the acceleration sensor utilizing an MEMS element, it is preferable that the capacitance of the MEMS element is large. To increase the capacitance of an MEMS element, the size of the lower electrode 92′ and movable portion 93a′ needs to be increased, and the gap between the lower electrode 92′ and the movable portion 93a′ needs to be reduced. However, to increase the size of the lower electrode 92′ and movable portion 93a′, the size of the sacrifice layer 94′ also needs to be increased. As a result, the time required for removing the sacrifice layer 94′ increases, which deteriorates the manufacturing efficiency of the MEMS element. Further, when the gap between the lower electrode 92′ and the movable portion 93a′ is reduced, the possibility that the movable portion 93a′ adheres to the lower electrode 92′ due to the surface tension of the etchant increases. In this way, when the measurement accuracy of an acceleration sensor is intended to be enhanced, various problems are caused in the process of manufacturing the MEMS element.
The above-described problems occur also in the process of manufacturing an actuator using the MEMS element shown in FIG. 20. Specifically, in an actuator utilizing the MEMS element, the movable portion 93a′ is driven by applying a voltage between the lower electrode 92′ and the upper electrode 93′. To enhance the operation accuracy of the movable portion 93a′, the size of the lower electrode 92′ and movable portion 93a′ needs to be increased, and the gap between the lower electrode 92′ and the movable portion 93a′ needs to be reduced. However, when the size of the lower electrode 92′ and movable portion 93a′ is increased, the manufacturing efficiency of the MEMS element is deteriorated. Further, when the gap between the lower electrode 92′ and the movable portion 93a′ is reduced, these parts may adhere to each other.